There are infrared imaging systems which have a lens assembly that images incoming infrared radiation onto an infrared detector. As the radiation travels from the lens assembly to the detector, it passes through a rotating chopper, and the chopper modulates the radiation by influencing it in a manner which varies as a result of the rotation of the chopper. For example, the chopper may alternately cause the radiation passing through it to be diffused to effect blurring, and permit the radiation to pass through it without any significant change.
One existing type of chopper has a frame that supports a polymer film. The polymer film has different portions which respectively include and are free of diffusion structure. Rotation of the chopper causes these portions to influence radiation passing through the chopper in an alternating manner. The film is a polymer material which is processed so that the polymer chains have a generally random orientation. The random orientation of the polymer chains has the effect of softening the material of the film to a point where, in order to keep the film flat, the film must be constrained to the frame at its perimeter, and then stretched taut within the frame. While existing choppers of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
For example, systems using this type of chopper are often expected to operate in a satisfactory and reliable manner across a relatively wide operational temperature range. However, the frame and the film have different coefficients of thermal expansion. As a result, temperature variations tend to induce tensile stresses within the film, due in part to the fact that the film is constrained at its perimeter. As a result, when the film has been subjected to several cycles of temperature variation, the tensile stresses often cause the film to tear, which in turn leads to degradation in the chopper's performance.